The present invention relates to processing of electrical signals, and more particularly to the filtering of a signal pertaining to at least one electrical property of an organ of a subject.
Technologies related to measurement of electrical properties of organs, such as the measurement of bioimpedance are generally known. Typically, such technologies relate to the monitoring of physiological parameters by extracting physiologically significant characteristics from electrical measurements, see, e.g., U.S. Pat. No. 6,577,897. Characteristics may include measures that aid in the discernment of physiological indications pertaining directly or indirectly to the state of organs (e.g., blood vessels, heart, lungs and the like), and reveal measures of various physiological conditions including critical life-threatening conditions.
For example, heart diseases may be caused by (i) a failure in the autonomic nerve system where the impulses from the central nervous system control to the heart muscle fail to provide a regular heart rate and/or (ii) an insufficient strength of the heart muscle itself where even though the patient has a regular heart rate, its force of contraction is insufficient. Either way, the amount of blood or the rate at which the blood is supplied by a diseased heart is abnormal and it is appreciated that an assessment of the state of a patient's circulation is of utmost importance.
The simplest measurements, such as heart rate and blood pressure, may be adequate for many patients, but if there is a cardiovascular abnormality then more detailed measurements are needed.
Cardiac output (CO) is the volume of blood pumped by the heart during a time interval, which is typically taken to be a minute. Cardiac output is the product of heart rate (HR) and the amount of blood which is pumped with each heartbeat, also known as the stroke volume (SV). For example, the stroke volume at rest in the standing position averages between 60 and 80 ml of blood in most adults. Thus, at a resting heart rate of 80 beats per minute the resting cardiac output varies between 4.8 and 6.4 L per min.
A common clinical problem is that of hypotension (low blood pressure); this may occur because the cardiac output is low and/or because of low systemic vascular resistance. This problem can occur in a wide range of patients, especially those in intensive care or postoperative high dependency units. In these high risk patients, more detailed monitoring is typically established including measuring central venous pressure via a central venous catheter and continuous display of arterial blood pressure via a peripheral arterial catheter.
In addition to the above measurements, the measurement of cardiac output is useful. For example, when combined with arterial pressure measurements, cardiac output can be used for calculating the systemic vascular resistance. The measurement of cardiac output is useful both for establishing a patient's initial cardiovascular state and for monitoring the response to various therapeutic interventions such as transfusion, infusion of inotropic drugs, infusion of vasoactive drugs (to increase or reduce systemic vascular resistance) or altering heart rate either pharmacologically or by adjusting pacing rate.
Several methods of measuring cardiac output are presently known, representative Examples include the Fick method, described by Adolf Fick in 1870, the amount of oxygen taken up by the body during respiration and the difference in oxygen concentration between venous and arterial blood is used to calculate the cardiac output; the transoesophageal echocardiography (see, e.g., U.S. Pat. No. 6,142,941) in which cardiac output is derived from blood flow velocity (recorded via Doppler shift) cross-sectional area of the blood vessel and heart rate; and the compliance based method (see, e.g., U.S. Pat. No. 6,485,431) in which the compliance of the arterial system is determined from measured arterial pressure and used for calculating the cardiac output as the product of the mean arterial pressure and compliance divided by a time constant. Also known are catheter based methods such as thermodilution (see, e.g., U.S. Pat. No. 4,153,048).
A non-invasive method, known as thoracic electrical bioimpedance, was first disclosed in U.S. Pat. No. 3,340,867 and has recently begun to attract medical and industrial attention [U.S. Pat. Nos. 3,340,867, 4,450,527, 4,852,580, 4,870,578, 4,953,556, 5,178,154, 5,309,917, 5,316,004, 5,505,209, 5,529,072, 5,503,157, 5,469,859, 5,423,326, 5,685,316, 6,485,431, 6,496,732 and 6,511,438; U.S. Patent Application No. 20020193689]. The thoracic electrical bioimpedance method has the advantages of providing continuous cardiac output measurement at no risk to the patient.
Various methods employing bioimpedance are found in: International Patent Application Publication Nos. WO2004098376 and WO2006087696, U.S. Pat. Nos. 6,022,322, 5,615,689 and 5,642,734, and U.S. Published Application Nos. 20030120170, 20060085048 and 20060122540, the contents of which are hereby incorporated by reference.